1. Field of the Invention
The present invention relates to an image forming apparatus, which uses dark-color and light-color developer materials, and to a method of controlling this apparatus.
2. Description of the Related Art
There is growing demand for further improvements in image quality in recent models of electrophotographic-type image forming apparatus. In particular, there is demand for the ability to faithfully reproduce photographs and security-patterns (which prevent unauthorized copying), etc., in addition to the usual text and graphs, on a printing medium. Although text requires a high resolution and photographs and security-patterns require rich tonality, it is difficult to satisfy both requirements simultaneously in the prior art.
In order to satisfy these requirements, inventions that make joint use of light-color toner and dark-color toner have been proposed (Japanese Patent Laid-Open Nos. 5-35038, 2000-231279, 2000-347476 and 2001-290319).
An electrophotographic image forming apparatus is not only capable of forming patterns on plain paper but can also form security-patterns and other images (text, lines and photographs, etc.) on plain paper.
When an image that has a mixture of a text/line region or equal-density large-area region, a photographic region and a security-pattern region is formed, a problem in image quality arises in an adjacent region, namely a region in which these different types of regions are adjacent to each other. It should be noted that an equal-density large-area region refers to a region in which the same pixel continues for at least a prescribed number of pixels in the laser scanning direction and for at least a prescribed number of pixels in the direction perpendicular to the laser scanning direction. For example, in case of a resolution of 600 dpi, a region (2.15 mm×2.15 mm) in which the same pixel continues for 50 or more pixels in the laser scanning direction and 50 or more pixels in the direction perpendicular to the laser scanning direction is an equal-density large-area region.
In order to develop a security-pattern, which must not be conspicuous, using a dark-color toner, it is necessary to reduce the size of one dot. In view of the characteristics of electrophotography, however, the reproducibility of one dot declines in this case and the size of a single dot tends to fluctuate. This results in the occurrence of unevenness in the security-pattern region.
On the other hand, it has also been considered to form one dot of a security-pattern in the maximum size and with a light-color toner. If the security-pattern and text/line region or equal-density large-area region are adjacent in this case, it is difficult to distinguish between the two (see FIG. 4). Further, it has been considered to form part of a photographic region using a light-color toner. If the photographic region and a text/line region or equal-density large-area region are adjacent in this case, it is difficult to distinguish between the two (see FIG. 5).
The specification of Japanese Patent Laid-Open No. 10-198137 proposes a method in which when a high-contrast image and a low-contrast image are formed adjacent to each other using a single developer, a white streak that occurs on the low-contrast side of the boundary is prevented. Specifically, an unexposed region is provided at a boundary that exists between a solid portion and a halftone portion. Since the unexposed region functions as a potential wall, white streaks can be prevented.
The specification of Japanese Patent Laid-Open No. 2003-089238 proposes a method of correcting image data, which corresponds to the edge portions of images, in order to prevent white streaks. It should be noted that the inventions described in the specifications of Japanese Patent Laid-Open Nos. 10-198137 and 2003-089238 are directed to problems relating to a single developer and are not directed to problems relating to developers of different densities belonging to the same hue.